Autonomous regional satellite based navigation systems have enabled several countries to cover their territorial footprint and the footprint of their surrounding areas. A regional satellite based navigation system such as a global navigation satellite system (GNSS) caters to the needs of specific users, for example, military personnel for military applications, and civilian users for civilian applications. For example, India is planning to deploy an autonomous regional satellite navigation system, namely, the Indian regional navigational satellite system (IRNSS), for surveying, telecommunication, transportation, identifying disaster locations, public safety, etc. The purpose of this navigation system is to cater to the needs of both standard positioning service (SPS) users and also to the needs of restricted service (RES) users. The SPS and the RES modes of service are dual services supported by any GNSS catering to civilian and military requirements exclusively. A military signal is generally acquired in two modes, namely, direct and indirect. The indirect mode is typically known as SPS assisted. There is a need for generating navigation data structures that address the needs of the SPS and RES modes of service. Specifically, for the RES signal, there is a need for the navigation data structure to account for the direct mode of acquisition or the SPS assisted mode. In the SPS assisted mode, with assistance from time of week (TOW) data, the jump on to the RES signal is established. The IRNSS will deploy a satellite constellation comprising seven satellites, three of which will be in geostationary orbits and four in geosynchronous orbits. The signals will be transmitted by the satellites in two frequency bands, namely, L5 frequency band (1176.45 megahertz (MHz)) and S1 frequency band (2492.08 MHz). The SPS signal will be modulated by a 1 MHz binary phase shift keying (BPSK) signal, whereas the PS signal will use a binary offset carrier, BOC (5, 2).
A number of issues need to be addressed while designing a satellite navigation system, for example, sensitivity improvements, jamming margins, robustness towards spoofing, multipath related improvements, time to first fix (TTFF), etc., for ensuring the efficiency and robustness of the satellite navigation system. Time to first fix (TTFF) is an important parameter that needs to be optimized in most satellite navigation receivers. TTFF is a measure of the time taken by a satellite navigation receiver to acquire satellite signals and navigation data, and output a first position solution, referred to as a “fix”, from power-on. The TTFF parameter directly influences the efficiency of position tracking by the satellite navigation receiver. The TTFF parameter has been examined at length and several approaches have been proposed to reduce this parameter. However, most of the approaches have concentrated on augmenting the satellite navigation receiver with data aid to the satellite navigation receiver. For an optimal TTFF performance, it is necessary that the time taken for computing navigation measurements and collecting subsequent navigation data is minimal. Typically, methods for reducing TTFF have focused on reducing the time required to acquire and lock the navigation signal, assisting the satellite navigation receiver with navigation data on a separate satellite link, etc. However, these methods are generally expensive in terms of deployment costs, complexity of the satellite navigation receiver, etc.
A typical global navigation satellite system (GNSS) signal may be characterized by the following equation:s(t)=c(t)*[r(t)⊕d(t)]
where the parameter s(t) refers to an output GNSS signal at a time instant t, the parameter r(t) refers to a ranging code at the time instant t, the parameter c(t) refers to a frequency of operation at the time instant t, and the parameter d(t) refers to the navigation data transmitted by each satellite.
The navigation data transmitted by each satellite can be grouped into ephemeris data and almanac data. The ephemeris data comprises precise clock and Keplerian parameters, which are typically updated once every two hours. Typically, the ephemeris data or ephemerides are transmitted periodically once every two hours. The almanac data provides a coarse estimate of a satellite orbit, which is used for satellite visibility computations. The almanac data also comprises ionosphere delay estimation coefficients for single frequency users, for example, global positioning system (GPS) L1 users. The almanac data typically changes once in a day. The satellite state vectors of a satellite computed using the ephemeris data are used for estimation of a user position and velocity.
The satellite navigation receivers of conventional satellite navigation systems have generally been constrained by the amount of time taken for collecting the ephemeris data and the almanac data that constitute the navigation data. The delay in collecting the navigation data translates to multiple delays, for example, delays in computing satellite visibility, delays in estimation of ionosphere delay estimation coefficients, delays in cross-correlation detection based on the range estimated using the almanac data and an integrity check specified by the federal aviation administration (FAA) for beta-3 civil aviation receivers, etc. Conventional satellite navigation receivers take a relatively long time, for example, about 12.5 minutes to collect the almanac data for a single frequency user. This delays the estimation of the ionosphere error, which is an important parameter for estimation of the position of the satellite.
The typical time taken by a user in open sky conditions to collect ephemeris data and almanac data from the global positioning system (GPS) and the global navigation satellite system (GLONASS) is recorded. The ephemeris data collection time in a GLONASS is, for example, about 30 seconds, and the ephemeris data collection time in a GPS is, for example, about 30 seconds. The almanac data collection time in a GLONASS is, for example, about 150 seconds, while the almanac data collection time in a GPS is, for example, about 750 seconds.
Furthermore, a conventional satellite navigation system such as the global positioning system (GPS) transmits the navigation (NAV) data, for example, as “sub-frames”, while the GLONASS transmits the NAV data, for example, as “strings”. In the GPS, Keplerian parameters are transmitted as a part of the navigation data while in the GLONASS, the absolute state vectors of a satellite are transmitted. Existing GPS based systems employ an L1 sub-frame structure, with the first three sub-frames comprising the ephemeris data and the last two sub-frames dedicated to the almanac data. Each sub-frame contains 10 words and each word has 24 navigation data bits and 6 parity bits. The use of 6 parity bits per word translates to 60 parity bits per sub-frame, effectively constraining the data bandwidth and delaying the time to first fix. The data bits are transmitted at 50 bits per second (bps). Therefore, one complete sub-frame is transmitted in 30 seconds. For every 24 bits, six redundant bits are transmitted. This constrains the time taken for the collection of the ephemeris data and delays the TTFF.
Furthermore, in each sub-frame, existing words and bits, for example, telemetry (TLM) data, hand over word (HOW) data, a sub-frame identifier, etc., need to be transmitted. The almanac data is transmitted in two sub-frames, for example, sub-frames 4 and 5. Moreover, the almanac data comprises ionosphere correction terms and coordinated universal time (UTC) parameters. Further, in case of current almanac transmission methods deployed in a GPS, at any given instant of time, all satellites transmit the same information as part of sub-frames 4 and 5. The sub-frames 4 and 5 transmit almanac data for all the 25 pages with each almanac page comprising the almanac data of a particular satellite. Furthermore, with the current scheme of almanac data transmission, as for example in GPS based systems, it takes about 168 seconds for a seven-satellite satellite navigation system to collect almanac data. This delays the ionosphere error computation and thus delays accurate positioning in a satellite navigation receiver. Moreover, parameters such as UTC parameters compound the delay and bandwidth overhead since the UTC parameters need not be transmitted very frequently for computation of the user's position.
A navigation (NAV) data structure of the Galileo GNSS adopts a sub-frame architecture similar to that of a GPS based system. The navigation data structure uses a 12 sub-frame structure with each sub-frame comprising a series of pages. Each page comprises a synchronization pattern and navigation data symbols. Each navigation data symbol comprises a navigation data word and tail bits. The navigation data word comprises a 24-bit cyclic redundancy check (CRC) code. Further, as in the case of other modern global navigation satellite systems (GNSSs), the Galileo adopts half rate forward error correction (FEC) encoding. For the Galileo GNSS, the integrity bits are added to a packet of navigation data. However, Galileo uses a navigation data structure with a larger number of sub-frames and imposes constraints in terms of memory requirements and an increased amount of time required for transmitting the complete navigation data structure.
The GPS L5 satellite navigation system is a global navigation satellite system that employs navigation data transmission based on the transmission of text messages at a predefined rate. Each text message is identified based on a message identifier (ID). GPS L5 uses a half rate forward error correction (FEC) encoding scheme with a baud rate of 100 symbols per second (sps). The signal transmitted from an L5 satellite is at a power level of, for example, about −157 decibel-watt (dBW). The GPS L5 satellite navigation system allows variation of frequency of text message transmission. However, the GPS L5 system continues to employ a five sub-frame structure and needs about 30 seconds for complete transmission of all the sub-frames of the navigation data structure.
Furthermore, the transmission of almanac data in a conventional global navigation satellite system (GNSS), for example, a GPS based system comprises transmission of the same almanac data by all satellites in a constellation. For example, in a seven-satellite constellation employed by the Indian regional navigational satellite system (IRNSS), each satellite transmits the same almanac data at each time instant over a satellite channel. This increases the time overhead in almanac transmission and increases the almanac data collection time at the satellite navigation receiver, thereby delaying ionosphere estimation at the satellite navigation receiver. For example, in the IRNSS, seven almanac pages need to be transmitted as a part of the third sub-frame. On using an almanac transmission scheme typically used in a GPS, it takes about 168 seconds to completely transmit the almanac data.
The number of physical channels within a receiver is not a constraint due to improvements in semiconductor technology. Several receiver manufacturers have developed receivers with an excess of 200 channels, which exist concurrently. In addition, the receivers support all in view global navigation satellite system (GNSS) satellite signal processing. The modernized signals of a global positioning system (GPS), namely, L2C and L5, and the proposed signals of Galileo and Compass navigation systems have a minimum of at least two frequencies that support civilian applications. In parallel, there exists dedicated access to their military applications. With an assumption of dual frequency, there is a need for reducing the TTFF for civilian applications, and more importantly, for the restricted users.
A study was carried out on a signaling scheme of operational navigation systems with respect to multiple frequencies of operation. Of the parameters used to compute TTFF, collection time of ephemeris data (Teph) is a major contributor as Teph completely depends on the navigation data structure of a particular constellation and does not depend on the satellite navigation receiver. As such, the TTFF is governed by the time required to collect ephemeris data. The objective is to reduce the Teph further without increasing the satellite data rate, the power required for data transmission, etc., and thus improve TTFF. An increased data rate necessitates more signal transmission power, which is a costly proposition onboard satellites.
Hence, there is a long felt but unresolved need for a method and a system that generate a navigation data structure with a few sub-frames, for example, three sub-frames, configure the navigation data in these sub-frames to selectively accommodate navigation data, and selectively transmit the generated navigation data structure with the configured navigation data to a satellite navigation receiver in reduced time to enable faster access to the navigation data and to reduce the TTFF for civilian users and restricted users.